4,4′-diaminodicyclohexylmethane, also known as hydrogenated diaminodiphenylmethane, is abbreviated as 4,4′-H12MDA or PACM. The main use of H12MDA is to produce methylenebis(4-cyclohexylisocyanate) (H12MDI) with superior properties, which is suitable for preparation of light and stable polyurethane coatings and paints, in the field of a new generation of anti-aging polyurethane. H12MDA can also be used as a curing agent for epoxy resins.
Generally, 4,4′-H12MDA is prepared by subjecting 4,4′-diaminodiphenyl methane (MDA) as a raw material to hydrogenation at a high temperature and under a high pressure in the presence of a catalyst, and then subjecting the reaction product to separation and purification. The reaction equation is as follows.

Due to the different configurational substituted amino group, 4,4′-H12MDA has three configurational isomers, i.e. cis-cis isomer, cis-trans isomer, and trans-trans isomer. At thermodynamic equilibrium state, H12MDA isomers show a distribution of 50% of the trans-trans isomer, 43% of the cis-trans isomer and 7% of the cis-cis isomer. Among the three isomers, the trans-trans isomer is the most thermodynamically stable. A high-temperature reaction will increase the trans-trans isomer. Generally, after the hydrogenation reaction, the final products contain a mixture of the three isomers.
H12MDA containing about 20% of the trans-trans isomer is referred to as PACM-20, and H12MDA containing about 50% of the trans-trans isomer is referred to as PACM-50. PACM-20 is the kind of H12MDA that is required in the field of the polyurethane industry. Due to the complicated and expensive separation process of the three configurational isomers, it is desirable to directly obtain H12MDA with a low amount of the trans-trans isomer. For this purpose, a lot of studies have been done around the selection and modification of catalysts and the optimization of reaction conditions.
European Patent EP0324190 describes that the hydrogenation can be performed at 50-350 bar and 100-190, provided that the supported catalyst has a BET specific surface area in the range of 70-280 m2·g−1 and an average pore diameter (dp) in the range of 10-320 Å, and the catalyst contains 0.1-5 wt % of ruthenium and has a penetration depth of at least 50 μm. The hydrogenated product contains 15%-40%, usually 20%-24% of the trans-trans isomer.
U.S. Pat. No. 4,394,523 provides a general method for producing H12MDA with a low amount of the trans-trans isomer. In this method, ruthenium supported on alumina is used as a catalyst, and the hydrogenation is performed in the presence of an aliphatic alcohol and ammonia, under H2 pressure of at least 36.5 bar. The product contains 15%-40%, usually 23%-30% of the trans-trans isomer.
U.S. Pat. No. 3,959,374 provides a two-step method for producing H12MDA by hydrogenation of a raw material containing a small amount of impurities. It is characterized by a pre-hydrogenation treatment of the raw material with a nickel catalyst which is relatively cheap, followed by a second hydrogenation with a ruthenium catalyst which is relatively expensive. The ruthenium catalyst can overcome the defects of low effect, and long reaction time of the nickel catalyst; however, it is easily poisoned by the impurities in the raw material, resulting in rapid activity deterioration. Thus, the method with the pre-hydrogenation treatment by the nickel catalyst can reduce the poison of ruthenium catalyst due to the impurities in the raw material, facilitate maintenance of the activity of ruthenium catalyst, ensure reasonable reaction time, and thereby avoid the increase of the amount of thermodynamically stable trans-trans isomer resulted by reacting at high temperature for a long time.
U.S. Pat. No. 4,754,070 provides a method for producing H12MDA with a low amount of the trans-trans isomer by using rhodium and ruthenium bimetal-supported catalyst. Due to the high activity of rhodium catalyst, H12MDA with a low amount of the trans-trans isomer can be prepared under mild reaction conditions. By this method, the amount of the trans-trans isomer is about 14%-28%.
Chinese patent CN101050184 provides a method for producing H12MDA by using a nanometer ruthenium-supported catalyst. It reports that the special preparation method for the catalyst allows ruthenium particles to be highly dispersed on the surface of the carrier, resulting high activity of the catalyst. The product contains 14%-27%, usually 20%-23% of the trans-trans isomer.
European Patent EP1251119 provides a continuous process for producing H12MDA, in which the conversion rate of MDA is no less than 95%, preferably no less than 99%. This patent seeks to increase the production capacity. However, as it uses multiple sets of tandem suspension reactors, the problem is that the operation of the equipment is complex.
N-methyl-diamino-dicyclohexylmethane (N—CH3—H12MDA), even in trace amount in H12MDA can greatly reduce the quality of the corresponding H12MDI. Thus, the amount of N—CH3—H12MDA in the H12MDA product must be reduced. N—CH3-4,4′-MDA can be hydrogenated in the same way as 4,4′-MDA so as to be converted into N—CH3—H12MDA. Due to the similar characteristics between N—CH3—H12MDA and H12MDA, a quite complex device has to be used to perform the separation, and thereby the yield of 4,4′-H12MDA is greatly reduced.
U.S. Patent US20050148797 proposes that the difficulty in separating N—CH3—H12MDA and H12MDA can be decreased by controlling the reaction process, i.e. controlling the conversion rate in a range of 90%-99%, to allow most of N—CH3-4,4′-MDA as an impurity in a raw material not to be hydrogenated. In this patent, three tandem fixed bed reactors are used as the reaction system. However, it neither provides intuitive technical indicators to generally control the reaction process (only analyzing samples by chromatography), nor involves how to control the content of the trans-trans isomer in the product, and nor provides any method to deal with the un-reacted raw material.
Chinese Patent CN101429139 provides a method, in which the contents of impurities in a raw material and intermediates are initially controlled by separation and purification of the raw material so as to decrease the catalyst poison by the impurities, elongate the life of the catalyst and decrease the production cost.
In view of the above, none of the methods provide a method for producing H12MDA with a low amount of the trans-trans isomer by controlling the reaction process and reducing the contact period between the catalyst and raw material at a high temperature, and none of them provide a technical solution for further treatment of un-reacted raw material.